Semiconductor wire array structures, and solar cells and photodetectors based on such structures

ABSTRACT

A structure comprising an array of semiconductor structures, an infill material between the semiconductor materials, and one or more light-trapping elements is described. Photoconverters and photoelectrochemical devices based on such structure also described.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. provisional application61/265,306 for “Light-trapping Si wire-array structure for solar cellsand photodetectors” filed on Nov. 30, 2009, and U.S. provisionalapplication 61/265,297 for “Selective p-n junction fabrication techniquefor high-aspect-ratio semiconductor microstructures” filed on Nov. 30,2009, and U.S. provisional application 61/313,654 for “Processing Stepsfor the Fabrication of a Microwire Array Solar Cell” filed on Mar. 12,2010, all three of which are herein incorporated by reference in theirentirety. The present application is also related to U.S. patentapplication Ser. No. 12/956,422 for “Three-dimensional patterningmethods and related devices” filed on even date herewith, alsoincorporated herein by reference in its entirety.

STATEMENT OF GOVERNMENT GRANT

This invention was made with government support under Grant NumbersDE-SC0001293 and grant DE-FG02-07ER46405 awarded by the U.S. Departmentof Energy. The US government has certain rights in the invention.

FIELD

The present disclosure relates to semiconductor microstructures, such ashigh-aspect-ratio of semiconductor microstructures. More in particular,the present disclosure relates to semiconductor wire array structures,such as silicon (Si) wire arrays structures, especially for solar cellsand photodetectors.

BACKGROUND

Solar cells based on arrays of Si micro- or nanowires have been proposedas a potentially low-cost alternative to conventional wafer-based Sisolar cells. See reference [1], incorporated herein by reference in itsentirety.

Device physics modeling, based on experimentally measured properties ofSi wires, has predicted that wires of micron-scale diameter will achievethe greatest photovoltaic energy conversion efficiency. See reference[2], incorporated herein by reference in its entirety. Such solar-cellstructure should effectively absorb all above-bandgap incident sunlight,over a broad range of incidence angle.

SUMMARY

According to a first aspect of the disclosure, a structure is provided,comprising: an array of elongated semiconductor elements; an infillmaterial located in a space between the elongated semiconductorelements; and a reflective material, configured to reflect incidentlight and direct the incident light to the elongated semiconductorelements.

According to a second aspect of the disclosure, a structure is provided,comprising: an array of elongated semiconductor elements; an infillmaterial located between the elongated semiconductor elements; and anantireflective coating at least partially and superficially covering arespective elongated semiconductor element, the antireflective layerbeing interposed between the infill material and the respectiveelongated semiconductor element.

According to a third aspect of the disclosure, a structure is provided,comprising: an array of elongated semiconductor elements; an infillmaterial located in a space among the elongated semiconductor elements;and a light scattering material included in the infill material andsurrounding the elongated semiconductor elements.

According to a fourth aspect of the disclosure, a structure is provided,comprising: an array of substantially vertically oriented elongatedsemiconductor elements; an infill material located in a space among theelongated semiconductor elements; and a material applied at leastpartially on a surface of the infill material, said material beingselected from the group consisting of: light scattering material,concentrating material, and a texture.

According to further aspects of the disclosure, solar cells,photoconverter devices and photoelectrochemical devices comprising theabove structures are also provided.

Further aspects of the disclosure are shown in the specification,drawings and claims of the present application.

Appendix 1, Appendix 2, and Appendix 3 are filed together with thepresent application and form integral part of the specification of thepresent application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic partial cross sectional view of a baselinestructure including a wire array.

FIG. 2 shows a schematic partial cross sectional view of a structureaccording to an embodiment of the present disclosure.

FIG. 3 shows a schematic partial cross sectional view of a structureaccording to another embodiment of the present disclosure.

FIG. 4 shows a schematic partial cross sectional view of a structureaccording to a further embodiment of the present disclosure.

FIG. 5 shows a schematic partial cross sectional view of a structureaccording to a further embodiment of the present disclosure.

FIG. 5 shows a schematic partial cross sectional view of a structureaccording to yet another embodiment of the present disclosure.

FIG. 6 shows a schematic partial cross sectional view of a structureaccording to still another embodiment of the present disclosure.

FIG. 7 shows a schematic partial cross sectional view of a structureaccording to a further embodiment of the present disclosure.

FIG. 8 shows a schematic partial cross sectional view of a structureaccording to a further embodiment of the present disclosure.

FIG. 9 shows a schematic partial cross sectional view of a structureaccording to a further embodiment of the present disclosure.

FIG. 10 shows a schematic partial cross sectional view of a structureaccording to another embodiment of the present disclosure.

FIG. 11 shows a schematic partial cross sectional view of a structureaccording to yet another embodiment of the present disclosure.

FIG. 12 shows a schematic partial cross sectional view of a structureaccording to still another embodiment of the present disclosure.

FIG. 13 shows a perspective view of a solar cell according to a furtherembodiment of the present disclosure.

FIG. 14 shows a schematic partial cross sectional view of a solar cellaccording to a further embodiment of the present disclosure.

FIG. 15 shows a schematic partial cross sectional view of aphotoelectrochemical device in accordance with the present disclosure.

FIG. 16 shows a schematic partial cross sectional view of a structureaccording to a further embodiment of the present disclosure.

FIG. 17 shows a schematic partial cross sectional view of a structureaccording to a further embodiment of the present disclosure.

FIG. 18 shows a schematic partial cross sectional view of a structureaccording to a further embodiment of the present disclosure.

FIG. 19 shows a schematic partial cross sectional view of a structureaccording to a further embodiment of the present disclosure.

FIG. 20 shows a schematic partial cross sectional view of a structureaccording to a further embodiment of the present disclosure.

FIG. 21 shows a schematic partial cross sectional view of a structureaccording to a further embodiment of the present disclosure.

FIG. 22 shows a schematic partial cross sectional view of a structureaccording to a further embodiment of the present disclosure.

DETAILED DESCRIPTION

With reference to FIGS. 1-22, the present disclosure describesstructures, solar cells, photoconverts and/or photoelectrochemicaldevices comprising an array of semiconductor structures, an infillmaterial between the semiconductor materials, and one or more of thefollowing light-trapping elements: textured surfaces,light-concentrators, light-scatterers, antireflective layers, orreflective layers; or combinations of such elements. These lighttrapping elements can be opportunely located in the structures to directincident light to the semiconductor materials. These light trappingelements can furthermore provide, in accordance with some embodiments,one or more of the following synergistic functionalities tophotoconverters based on these structures: passivation of thesemiconductor surfaces, conduction of electrical current, or structuralsupport.

FIG. 1 shows a schematic partial cross sectional view of a baselinestructure (101) including an array of elongated semiconductor elements(110), such as Si wires. In general, FIGS. 1-12 and 16-22 show schematicpartial cross sectional views of structures (102), (103), (104), (105),(106), (107), (108), (109), (1001), (1002), (1003) and furtherstructures (1004) (1005), (1006), (1007), (1008), (1009) and (1010),according to respective embodiments of the present disclosure. All ofthe structures (102), (103), (104), (105), (106), (107), (108) (109),(1001), (1002), (1003), (1004), (1005), (1006), (1007), (1008), (1009)and (1010) can include the baseline structure (101) of FIG. 1.

The baseline structure (101) comprises an array of wires (110), such aselongated semiconductor elements, which, by way of example and not oflimitation, is a square-tiled array of 67-μm-long Si wires (110) havingan areal packing fraction of ηf=4.2%. In such specific embodiment, thestructure (101) contains the same volume of Si as a 2.8 μm-thick planarsheet of Si. Such array (101) shows peak absorption at normal incidence(<0.5), and increased absorption at steeper angles of incidence. Thewires can have any kind of shape, such as a cylindrical shape and havethe same shape. In other embodiments of the disclosure, the wires arecones, pyramids, wires or whiskers. Moreover, the semiconductor wires(110) can be tiled according to an ordered lattice pattern within thearray. In a particular embodiment, the semiconductor wires are tiled tohave a triangular, square, chimed triangular, chimed square, penrose,dodecagonal, or quasi-random ordered lattice pattern. Moreover, in someembodiments, the semiconductor elongated elements can be coated withadditional materials for reasons other than the improvement of opticalabsorption.

The Si wires (110) extend from a substrate (112), for example a quartzslide, which is a well-suited substrate for optical transmission andreflection measurements. The Si wires (110) are embedded in atransparent casing, infill material or environment (114). The infillmaterial (114) is generally transparent and has an index of refractionthat is greater than 1.0 and less than that of the semiconductormaterial, including such materials such as a polymer casing, inparticular polydimethlysiloxane (PDMS), air, EVA (ethylene vinylacetate), liquids, oxides, mylar, or wax. In particular, the infillmaterial (114) is located in a space between the elongated semiconductorelements or Si wires (110). The infill material (114) is able toeffectively reduce the reflectivity of the semiconductor wires.

According to several embodiments, the structure (101) hasnon-subwavelength dimensions. The expression “subwavelength” is intendedto mean that the wires or wire-like or elongated semiconductor elements(shaped e.g. as cylinders, cones, pyramids, elongated solids,tree-shaped elements etc.) have average minimum dimension (e.g. radius,width, etc.) that is less than the free space wavelength of sunlight,for wavelengths of sunlight having photon energy in excess of thesemiconductor bandgap. In some embodiments, the elongated semiconductorelements have diameters of at least 1 microns, lengths of at least 20microns or more and length to diameter ratios of at least 5. Forsilicon, the structure (101) has an average minimum dimension less thanabout 1100 nm (1.1 microns). According to further embodiments, thestructure (101) includes non-Si elongated semiconductor elements, suchas, for example Ge, GaP, GaAs, InP, InGaAs, SiC.

In some embodiments, the structure (101) has a packing fraction lowerthan 10%. In other words, in some embodiments, the volume occupied bythe semiconductor material or wires forms less than 10% of the volume ofthe structure. In additional embodiments, the structure (101) has apacking fraction lower than 5%. In fact, the expression “packingfraction” or “aerial packing fraction” is defined as the relativepercentage (by volume) of semiconductor material within the arraystructure. The structure is intended as including the semiconductorwires and other materials (voids, casing, particles, coatings, etc.)lying between the horizontal planes that confine the upper and lowerextent of the wires and their coatings. As an example, an array ofvertical cylindrical semiconductor wires tiled in a square lattice canbe considered, in which each wire has height h and radius r and isspaced a distance 1 from adjacent wires. With reference to this example,the packing fraction is πr²/l². In some embodiments, a structureincluding the structure (101) is configured in such a manner that atleast 80% of visible light incident on the structure from one or moreangles of incidence is absorbed by the structure. More in particular, atleast 50% of photons from the solar spectrum having energy greater thanthe semiconductor bandgap energy that are incident on the structure fromone or more angles of incidence are absorbed within the semiconductormaterial. In some embodiments, the projected area of the semiconductorstructures occupy less than 10% of the optical incidence plane.

FIG. 2 shows a structure (102), which includes the structure (101) andfurther includes a back reflector (115), for example a metal layer, suchas a minor-like Ag back-reflector, interposed between the Si wires (110)and the substrate (112). According to further embodiments, othermetallic or dielectric back-reflectors, having specular or non-specular(e.g. Lambertian) reflectance, can be used instead of a minor-like Agback-reflector. The back reflector can be made, for example, of metalslike Al, Ag, Au, Cu, Ni, Ti, or In. In other embodiments, the backreflector (115) acts as substrate and entirely replaces the substrate(112), as shown in FIG. 17.

The back reflector (115) increases an optical path length within thearray (102). In particular, the presence of the back reflector (115)increases the optical path length for absorption within the wire array(102) (approaching peak normal-incidence values of 0.8). In thisembodiment, the normal-incidence absorption can remain weaker than thatat off-normal-incidence angles because there is no randomization oflight within the array. At normal incidence, light travels parallel tothe wires, hits the back reflector, and then travels vertically upwardagain to escape the array structure without striking the wires.

According to further embodiments, the back reflector can be positionedonly under the Si wires (110), or only under the infill material (114),or under both the Si wires (110) and the infill material (114). Itfollows that the back-reflector extend either partially over the entiresubstrate (112), as shown for example in the structure 1004 of FIG. 16(where the back reflector is beneath the infill only) or beneath theentire structure as shown in the structure 1005 of FIG. 17.

According to further embodiments, light-scattering material or surfacetexture (115 a) is applied on the back reflector (115). Suchlight-scattering material (115 a) is able to change the direction ofincident light upon reflecting from the back reflector in a manner so asto increase the optical path length within the structure, particularlyfrom incidence at angles normal to the reflector. In one embodiment, thestructure 1006, shown in FIG. 18, includes the back-reflector featuresrandom texture so as to produce Lambertian reflectivity. Suchlight-scattering causes light to strike the wires (110) that mightotherwise have traveled between the wires without being absorbed. In oneembodiment, such light-scattering is achieved through random surfacetexturing applied to the top surface of the infill material, as shown inthe structure (1008) of FIG. 20, whereas in another embodiment suchlight-scattering is achieved through a curvature of the infill topsurface near the wires due to surface tension, volumetric expansion orcontraction, or other means, as shown in the structure (1009) of FIG.22.

FIG. 3 shows a portion of a structure (103), which is similar to thestructure (102) of FIG. 2, and further includes a light-concentratingtexture (116) located on the upper surface of the infill material (114)and facing the incident light (L). Such light-concentrating material(116) serves to change the direction of incident light upon transmissioninto the infill material or wires in a matter that increases the averageoptical path length within the structure. According to furtherembodiments, light-concentrating material or surface texture (116) isapplied on the back reflector (115). Such light-concentrating materialacts as a focusing reflector that focuses the light towards the Si wires(110) and improves light absorption. In one embodiment suchlight-concentrating texture (116) is approximated by parabolic dishreflectors beneath each wire (110), as shown in the structure (1007) ofFIG. 19.

FIG. 21 shows a portion of a structure (1010), which is similar to thestructure (102) of FIG. 2, and further includes a light-concentratingtexture (116) located on the upper surface of the infill material (114)and facing the incident light (L). Such light-concentrating material(116) serves to focus the light (L) towards the Si wires (110) andimproves light absorption, while decreasing the light acceptance angleof the structure. The structure (1010) differs from the structure (103)of FIG. 3 for the absence of any substrate physically distinct from theback reflector. In FIG. 21 the back reflector (115) acts as substrate.

FIG. 4 shows a structure (104), which includes the structure (101) ofFIG. 1, and further includes an antireflective coating (118) depositedon each Si wire (110). In the depicted embodiment, the antireflectivecoating consists of amorphous silicon nitride (a-SiN_(g)), which alsoserves as a surface passivation layer on the Si surfaces. According tofurther embodiments, other materials can be used instead of a siliconnitride antireflective layer, which may or may not also serve as surfacepassivation. Such surface passivation and/or antireflective coating(118) is deposited on the top (120) and side (119) walls of the Si wires(110). According to further embodiments, the antireflective coating(118) partially covers the surfaces of the Si wires. In the embodimentof FIG. 4 the surface passivation antireflective coating (118) is, forexample, a-SiN_(x) AR-coating (e.g., 80 nm nominal thickness) and isdeposited on the Si wires (110) prior to embedding the Si wires (110) inthe infill or casing (114). According to further embodiments theantireflective coating (118) can include one or more of nitrogen,oxygen, hydrogen, and/or silicon.

According to further embodiments, the antireflective layer (118) variesin thickness along a surface of the elongated semiconductor elements,for example, to minimize reflection of certain wavelengths of light atvarious positions along the elongated semiconductor elements. For asimple quarter-wave antireflective coatings, the ideal layer thicknessdepends directly on the wavelength of light, which for solarapplications, can span from 280-4000 nm, as well as the refractive indexof the antireflective material. By varying the antireflective layerthickness along the surface of the elongated semiconductor structure,optimal antireflection for one particular wavelength can be achieved atone location along the structure, whereas optimal antireflection for adifferent wavelength can be achieved elsewhere along the structure.According to a further embodiment, the antireflective layer (118) variesin composition along a surface of the elongated semiconductor elements.In one embodiment, the antireflective layer consists of a transparentconductive oxide such as indium tin oxide at the tops of the elongatedsemiconductor elements, and a transparent dielectric such as siliconnitride along the sides of the elongated semiconductor elements. Thisconfiguration is beneficial for solar cells comprising arrays ofelongated semiconductor structures, wherein the conductive top-layerantireflective material also serves as a device electrode, and whereinthe dielectric sidewall antireflective material also serves as a surfacepassivation layer. In further embodiments, light scattering materialsare further applied on the antireflective layer (118). In someembodiments, the antireflective coating is produced by plasma-enhancedchemical vapor deposition.

The structure (104) of FIG. 4 also includes light scattering particles(123), for example Al₂O₃ particles (e.g., 0.9 μm nominal diameter). Suchparticles are added to the infill material (114) and laterally surroundthe Si wires (110), to scatter the light that might otherwise passbetween the Si wires (110). In particular, in some embodiments, thelight scattering particles (123) uniformly surround all of the sides(119) of the Si wires (110). According to further embodiments, insteadof the light scattering particles (123), the structure includeslight-scattering elements selected from the group including void,bubble, dielectric composition, metal particle, and a polymer. Inparticular, the dielectric composition comprises one element selectedfrom the group including Al₂O₃, BaSO₄, TiO₂, SiO₂ and the metal particlecomprises one element selected from the group including Ag, Au, Ni, Aland Cu.

In FIG. 5, the infill material (114) includes an infill bottom zone(117) and a different infill top zone (121). Concentration of the lightscattering elements (123) is greater in the infill bottom zone (117)than in the infill top zone (121). In other words, the light scatteringelements (123) are concentrated near the bottom of the wire array in azone opposite to the incident light, or near the substrate, and morediffused or less concentrated in a zone faced to the incident light. Inthis way, light that is reflected or scattered upwards has more distancewith which to interact with the wires, thus increasing the probabilityof absorption. Similar results can be obtained when density of the lightscattering elements (123) is higher in the infill bottom zone (117) thanin the infill top zone (121). These results can be obtained, forexample, by centrifugation to force the particles to the bottom. Inother embodiments, the light-scattering structure or material within theinfill material is varied in density or composition with proximity tothe semiconductor structures. In other words, concentration of the lightscattering elements (123) is higher in proximity to the elongatedsemiconductor elements (110) than in a far zone thus further increasingthe probability of absorption into the elongated semiconductor elements(110). In some embodiments, the light-scattering material within theinfill material (114) comprises the incorporation of a translucentmaterial, such as wax or a white polymer to increase the scatteringcapacity of the light-scattering material. In other embodiments,light-scattering structure within the infill material comprises voids ofthe material or bubbles of air. Moreover, in some embodiments, in orderto further increase the probability of absorption into the elongatedsemiconductor elements (110) the light-scattering material is applied tothe surface of the semiconductor, infill, antireflective, or reflectivematerials.

With continued reference to FIG. 4 and FIG. 5, the structure (104) andthe structure (105) can include a light-scattering material located onthe upper surface of the infill material (114) and facing the incidentlight. Such light-scattering material scatters the light towards the Siwires (110) and improves light absorption, as already described above.According to further embodiments, a light-scattering material is placedon the surface of the infill material.

Such surface passivation antireflective coating (118) and lightscattering particles (123) were chosen because they have negligibleabsorption across defined wavelengths of 500-1100 nm, and thus enable adirect observation of absorption enhancement within the Si wires (110)themselves. The surface passivation antireflective coating (118) andlight scattering particles (123) virtually eliminate an angularsensitivity of the wire array's absorption, and increase the peaknormal-incidence absorption to 0.92. This is desirable because maximalabsorption is desired at normal incidence for most solar applications.If the absorption is significantly less at normal incidence than forother angles, then a solar cell will produce less electricity when thesun is directly overhead, when there is the greatest potential toproduce solar energy.

FIG. 5 shows a structure (105) which is similar to the structure (104)of FIG. 4 and further includes the back reflector (115) such as the Agback-reflector shown in FIG. 2. It follows that the structure (105) ofFIG. 5 differs from the structure (104) of FIG. 4 for the presence ofthe back reflector (115).

It is noted that such presence of the back reflector determines, in someembodiments, an array's peak absorption increasing to 0.96, which isnearly the maximal absorption achievable by any material fully embeddedwithin an infill (114) such as PDMS due to the about 3% reflectivity ofthe PDMS-air dielectric interface.

FIG. 6 shows a structure (106), which includes the structure (101) ofFIG. 1, and further includes the previously described surfacepassivation antireflective coating (118), placed on each Si wire (110).The surface passivation antireflective coating (118) is deposited ontothe top (120) and side (119) walls of the Si wires (110).

FIG. 7 shows a structure (107), which includes the structure (101) ofFIG. 1, and further includes the previously described light scatteringparticles (123), for example Al₂O₃ particles (0.9 μm nominal diameter).Such particles are added to the infill and randomly surround the Siwires (110), to scatter the light that might otherwise pass between theSi wires (110). In particular, the light scattering particles (123)surround the Si wires (110) near the bottom.

FIG. 8 shows a structure (108) which is similar to the structure (105)of FIG. 5, with the difference that the coating (118) is located only onthe top of the elongated elements or wires (110), facing the incidentlight (L). Moreover, the structure (108) includes elongatedsemiconductor elements (110), which are randomly located and randomlyoriented. Moreover the elongated semiconductor elements (110) presentangled or angular or sharp profile. Both these features increase, incombination with light scattering material or texture material orreflective material, the probability of light absorption.

FIG. 9 shows a structure (109) according to a further embodiment of thepresent disclosure which includes an array of elongated semiconductorelements (110). The semiconductor elements (110) have top ends (120) andfree bottom ends (130). The semiconductor elements (110) are embedded inthe infill material (114) and the antireflective coating (118) islocated on a free bottom end (130) of the elongated semiconductorelements (110). The structure (109) further includes a back reflector(115) which is shaped as a concentrating lens (116). The concentratinglens (116) focuses some or all of the light (L) incident on thestructure (109) onto the smaller area occupied by the semiconductorwires.

According to a further embodiment of the present disclosure, a structurecomprises an array of elongated semiconductor wires, such as thestructure (101) disclosed above, wherein the semiconductor wires aregenerally oriented vertically; and an optical concentrator element, suchas, for example, the concentrating lens (116) mentioned above thatfocuses some or all of the light incident on the structure onto thesmaller area occupied by the semiconductor wires. According to furtherembodiments, light concentrators are placed on the semiconductor wires.In some embodiments, the concentrating lens (116) can be included in anyone of the embodiments of FIGS. 2-8, 10-12 and 16-22.

The focusing lens (116) can focus the light to one or more of thesemiconductor wires. According to a further embodiment, the concentratorelement is below or adjacent to one or more of the semiconductor wires.According to further embodiments, the average cross-sectional area ofthe semiconductor wires within the array comprises less than 10% of thecross-sectional area of the optical incidence plane. In particular, theabsorption of light (L) within the semiconductor material decreases toless than 50% of the value produced under normal-incidence illuminationfor illumination incidence angles exceeding 45° from normal.

The infill material can be textured to produce the focusing lens aboveone or more of the semiconductor wires. According to furtherembodiments, the focusing lens is coated with a reflective material toproduce a focusing reflector below or adjacent to one or more of thesemiconductor wires. The surface texturing can be produced by thecontraction of the infill material, surface tension of the infillmaterial, particles or voids within the infill material, imprintlithography, or casting. The semiconductor wires can be tiled accordingto an ordered lattice pattern within the array, and the ordered array offocusing lenses is positioned above the structure or an ordered array offocusing reflectors is positioned below the structure.

FIG. 10 shows a structure (1001), which includes an array ofsemiconductor wires or cones generally vertically oriented, wherein thediameter of one or more wires or cones is flared near either or bothends of the structure, to produce a feed horn structure including atleast one elongated portion (110 b) and one flared portion (110 a), or atruncated cone portion. The flared portion (110 a) faces the incidentlight (L). The average diameter of each wire or cone (110) issufficiently large to support guided optical modes within thesemiconductor material. Instead of the feed horn structure, thestructure (1001) can include a plurality of truncated cone wires.

According to further embodiments, the flared portion (110 a) can beopposite to the incident light (L). Additionally, the cross-sectionalarea of the larger end of the flared portion (110 a) can be at leastfour times the average cross-sectional area of the elongated portion ofthe semiconductor wire. The taper in the diameter of the feed hornstructure or of a truncated cone wire can provide total internalreflection of light incident into the wire at normal incidence.

The structure (1001) of FIG. 10 includes an antireflective layer (118)placed on the top of the infill material (114). Other trapping lightelements can be present, such as the focusing lens (116) and/or thelight scattering particles (123) and/or the back reflector (115), asalso those described in structures (102)-(109).

According to further embodiments, the feed horn structure, or eachtruncated cone wire, is produced by an increase or decrease in thediameter of a semiconductor wire or cone during its growth. The feedhorn structure can be produced by etching a semiconductor wire or of acone. The feed horn structure can produced at the bottom of asemiconductor wire or cone and then subsequently relocated to top of thestructure by removing the wire or cone from the original supportsubstrate and turning it upside-down.

FIG. 11 shows a structure (1002) according to a further embodiment ofthe disclosure. In particular, the structure (1002) includes wireshaving frusto-conical shape or feed horn structure as in the embodimentof FIG. 10, and a back reflector (115) placed under the wires (110). Alarger flared portion (110 a) of the frusto-conical wires (110) islocated opposite to the incident light (L).

FIG. 12 shows a structure (1003) according to a further embodiment ofthe present disclosure. In particular, the structure (1003) comprises anarray of semiconductor wires (110), which are tilted to a non-verticalorientation so as to increase the projected cross-sectional area of thesemiconductor wires within the optical incidence plane atnormal-incidence illumination. In the embodiment of FIG. 12, the wires(110) are generally oriented parallel to one another and share similardiameter and height.

According to further embodiments, the non-vertical orientation of thesemiconductor wires is produced by growing the wires on a substrate in amanner that yields non-vertical growth, or by etching the wires from asubstrate in a manner that yields non-vertical wires. According toalternative embodiments, the non-vertical orientation of thesemiconductor wires is produced by growing or etching the wires in amanner that yields vertical or near vertical wires, then embedding thewires within an infill material and exerting a sheer force such that thewires remain tilted at a non-vertical angle within the plane of thestructure.

The structure (1003) of FIG. 12 includes the back reflector (115) placedunder the infill material (114). Other trapping light elements can bepresent, such as the focusing lens (116) the light scattering particles(123), the antireflective coating (118), and/or those described instructures (102)-(109).

The structures (101), (102), (103), (104), (105), (106), (107), (108),(109), (1001), (1002), (1003), (1004), (1005), (1006), (1007), (1008),(1009) and (1010), or any combination of such structures, describedabove can be manufactured according to the technique described inreference [3], incorporated herein by reference in its entirety. Siwires (110) are grown on p type <111> Si wafers (ρ<0.001 ω·cm), using,for example, a 300 nm thermal oxide for catalyst confinement andevaporated Au, Cu, or Ni (400-700 nm thickness) as the VLS catalyst. Nonotable differences were observed between the optical properties ofwires grown using Au, Cu, or Ni catalyst metal.

Following growth, the wire arrays were etched in 5% HF(aq) for 30 s. Toremove the catalyst metal, Au-catalyzed wires were then etched for 30min in a solution of 9:1 Gold Etchant TFA (Transene) to 36% HCl(aq) andthen rinsed for 30 s in 5% HCl(aq). Cu- and Ni-catalyzed wires wereinstead etched for 20 min at 70° C. in a 6:1:1 solution of H2O:H2O2:HCl.Both groups of wires were then HF-etched as described above, dried, andmomentarily dipped in a 50% (wt) solution of KOH (aq) at 55° C., toremove ˜20 nm of Si, thus removing the metal-rich surface layer observedin similarly grown wires. For the structures including surfacepassivation antireflective coating (118), a SiN_(x) film of 80-nmnominal thickness was conformally deposited onto the wire arrays byplasma-enhanced CVD at 350° C. In some embodiments, the reflectivematerial comprises the substrate on which the elongated semiconductorstructures were grown, or the substrate from which the elongatedsemiconductor structures were etched.

The lengths, diameters, and areal fractions of each wire array can bedetermined by computer-processing of high-resolution SEM images, takenfrom a 200×200 μm area at the center of each array. Only near-perfectwire arrays, defined as those that had at most one defect within thisarea (e.g. non-vertical or spurious growth, or a wire missing from thepattern), were considered. Arrays were embedded in PDMS and peeled-off.

The PDMS can be, for example, drop-cast, spun at 3000 rpm, and thencured at 120° C. for ≧1 hr, resulting in a smooth film whose overallthickness ranged from 10 to 50 μm greater than the height of the wirearray. Subsequently, the wire arrays are transferred on the quartzslides for optical measurements. The arrays themselves are flexiblepolymer films, and could be transferred to any substrate (e.g. a window)or left as a free-standing flexible film. They have properties similarto a window decal.

For the structures which incorporate light scattering particles, e.g.Al₂O₃ light-scatterers, particles of 0.9 μm nominal diameter, whosesurfaces had been modified with trimethylchlorosilane, were dispersedinto CH₂Cl₂ by sonication. This solution was mixed into the PDMS toyield a ratio of 1:10:10 Al₂O₃:CH₂Cl₂:PDMS by weight. The suspension wasdrop cast, spun, and cured as described above. Prior to curing, thearrays were centrifuged for several minutes to drive the Al₂O₃ particlestowards the bottom of the PDMS layer. In particular, in someembodiments, light-scattering materials are placed between thesemiconductor structures before the infill material is added to thestructure. The light-scattering materials can be mixed into the infillmaterial before the infill is added to the structure. In someembodiments, the light-scattering property of the infill material isproduced by a chemical process following the addition of the infillmaterial to the structure. In some embodiments, the composition of theinfill material is varied as it is added to the structure to produce aninhomogeneous distribution of light-scattering materials or structureswithin the structure. In some embodiments the distribution oflight-scattering materials or structures is produced through evaporationor contraction of the infill material. For the structures including theback reflector, such back reflector is placed on the quartz slides usinga thermal evaporator. Each array is placed on a clear quartz slide or ametal-coated one to compare their absorption with vs. without the backreflector. In an envisioned solar cell (FIG. 8) the metal would bedeposited directly onto the wire arrays rather than onto quartz slides.In that case, it would also serve as a back metal contact. For structureincluding light-scattering texture such texture is produced by thecontraction of the infill material, surface tension of the infillmaterial, particles or voids within the infill material, imprintlithography, or casting.

To provide a figure of merit for the absorption measurements, theoverall fraction of above-bandgap photons that each wire array wouldabsorb throughout a day of operation as a non-tracking solar cell,A_(avg), was calculated based on a time-resolved reference spectrum ofdirect solar insolation, see reference [4], incorporated herein byreference in its entirety, in conjunction with the measured angle- andwavelength-dependent absorption values of the wire arrays of FIGS. 2-5.A_(avg) calculations that correspond to the absorption measurements arecompared with the A_(avg) calculation that corresponds to the measuredabsorption of a commercial, 280-μm-thick polycrystalline Si solar cellwith a dielectric AR-surface passivation antireflective coating. In aparticular embodiment, the optimal Si wire array (105) of FIG. 5exhibited A_(avg)=0.85, which although slightly below that of thecommercial Si solar cell (A_(avg)=0.87), is remarkable considering thatthis wire array film contained ˜1% as much Si (per specimen area) as thecommercial solar cell. This volume reduction implies substantial opticalconcentration within the Si wires.

To further gauge the absorption enhancement of the wire array geometry,the measured absorption, A_(WA)(θ_(x), λ), of the wire array (105) fromFIG. 5 was compared to the theoretical absorption limits of a“equivalently thick” (2.8 μm) planar Si absorber. Based on bulk Siproperties, see reference [5], incorporated herein by reference in itsentirety, and neglecting interference effects, two theoreticalabsorption limits were calculated for the equivalently thick Si slab: i)A_(Si), which results from the use of bare, non-textured Si surfaces(black); and ii) A_(LT), which results from ideal classicallight-trapping at the Si surfaces. The latter case, the “Ergodic limit,”is the maximally achievable absorption (in the ray-optic limit) of aplanar-sheet absorber that utilizes ideally random (e.g. Lambertian)light trapping, see references [6], [7], incorporated herein byreference in their entirety.

The wire array's absorption exceeds the planar light-trapping limit forinfrared wavelengths (λ>800 nm). This behavior exemplifies a usefulproperty of micro-structured, non-planar absorber geometries (includingwire arrays), in that they can achieve greater absorption per materialvolume than achievable by a randomly textured, planar-sheet absorbergeometry. This effect has been described, through use of a statisticalray optics model, for idealized films of polymer-embedded Si granules,see reference [7], and has also been simulated for Si wire arrays, seereferences [8] and [9], incorporated herein by reference in theirentirety. The enhanced infrared absorption of the Si wire array yieldeda greater overall absorption of above-bandgap photons than theequivalently thick, ideally light-trapping planar absorber. In fact,taking all measured incidence angles into account, the day-integratedabsorption of the wire array (A_(avg)=0.85) slightly exceeded that ofthe planar light-trapping case (A_(avg)=0.82). Thus, the Si wire arraygeometry can enable solar cells that reach, and potentially even exceed,the theoretical absorption limit, per volume of Si, of ideallight-trapping within a conventional planar geometry.

The enhanced absorption properties of Si wire arrays enable high quantumefficiencies for photovoltaic applications. To demonstrate this, aphotoelectrochemical cell was used to measure the external quantumefficiency (EQE) of Si wire-array photoelectrodes, which consisted ofp-type wire arrays grown on degenerately doped (and thusphotovoltaically inactive) Si wafers. The transparent electrolyte formeda rectifying junction to the top and sides of each wire (analogous to aradial p-n junction), enabling photoelectrochemical characterization ofthe angle- and wavelength-dependent EQE of the wire-array electrode.However, because the wires were immersed in an electrolyte and attachedto their growth substrate, this technique did not permit the use of apolymer infill, a dielectric antireflective coating and/or a planarmetal back-reflector. Thus, relatively long (130 μm) and sparse(η_(f)=6.2%) square-tiled wire arrays were grown, to minimize thetransmission of light into the photovoltaically inactive growthsubstrate while also minimizing the area of the reflective top surfaceof the Si wires. This geometry yielded up to 0.85 peak EQE, but sufferedfrom substantially reduced EQE at normal incidence. Evaluating the EQEacross the day-integrated solar spectrum (as carried out for A_(avg)above) yielded EQE_(avg)=0.56. When Al₂O₃ light-scattering particleswere drop-cast into this wire array, the normal-incidence ‘dead spot’was virtually eliminated, the peak EQE increased to 0.89 and theday-integrated EQE_(avg) increased to 0.68. This value is significant,considering that the photoelectrochemical cell configuration precludedthe use of a metal back-reflector or an antireflective coating, both ofwhich are known to substantially improve the optical absorption asdescribed above, and both of which could be used within a solid-state,radial p-n junction wire-array solar cell. Thus, the results describedhere represent lower bounds, rather than upper limits, on the EQE thatcould be achieved by use of the Si wire-array geometry making use of thelight-trapping elements disclosed herein. The configuration of theelectrode described in this paragraph is depicted in FIG. 15.

FIGS. 13 and 14 of the present application describe embodiments relatingto the applications, for example, of the structures (101), (102), (103),(104), (105), (106), (107), (108), (109), (1001), (1002), (1003),(1004), (1005), (1006), (1007), (1008), (1009) and (1010) or anycombination of such structures, to semiconductor wire-arrayphotovoltaics and photoconverters, i.e. devices that convert light intoelectricity, including solar cells, photoelectrochemical cells,photodiodes, phototransistors, and other photosensitive wire-arrayelectronic devices. Such semiconductor wire-array photovoltaics andphotoconverters can indifferently be based on any one of the structures(102), (103), (104, (105), (106), (108), (109), (1001), (1002), (1003),(1004), (1005), (1006), (1007), (1008), (1009) and (1010) or anycombination of such structures.

As already mentioned in the introductory paragraph of the presentapplication, the present application is also related to U.S. patentapplication Ser. No. 12/956,422 for “Three-dimensional patterningmethods and related devices” filed on even date herewith, incorporatedherein by reference in its entirety, and claiming priority to the sameUS provisional applications of the present application. Such as U.S.patent application Ser. No. 61/265,297 which describes selective p-njunction fabrication for semiconductor microstructures, and relatedmethods and devices.

In particular, shown in FIG. 13 is a perspective view of a Si wire arraysolar cell (600) with a transparent top contact (610) (e.g. an indiumtin oxide (ITO) contact) and a metallic bottom contact (620) acting as aback reflector. Solar cell (600) is embedded into a polymer encasing orenvironment (630), e.g. a flexible transparent polymer such as PDMS.Light scattering particles (640) (e.g., Al₂O₃ particles as describedbefore) are embedded inside the infill or environment (630), so as tosurround and circle the Si wires (101). The solar cell (600) furthercomprises an array of mechanically flexible vertically aligned wires(650). The wires (650) are coated with a surface passivationantireflective coating (660).

The device of FIG. 14 differs from the device of FIG. 13 for the wireswhich remain on a rigid growth substrate (612) and for which the backreflector (620) is placed between the wires on top of said growthsubstrate rather than forming a continuous film beneath the infill andsemiconductor structures. Furthermore, the bases or bottom of thesemiconductor structures, shown in FIG. 14, exhibit feedhorn-likestructures as described above and therefore includes a flared portion(110 a). Moreover, the device of FIG. 14 includes light scatteringmaterial (640) having a light scattering material concentration higherin a bottom zone (630 b) of the infill material than in a top zone (630a) of the infill material. In the present embodiment the back reflector(620) is the metal layer and the substrate (612) functions as thecontact. Three different types of Si microwire solar cells werefabricated. The “As-Grown” cell contained no light trapping elements orantireflective coating on the semiconductor surfaces. The “Scatterer”cell incorporated light-scattering Al₂O₃ particles (nominally 80 nm indiameter) in-between the wires. In a particular embodiment, Al₂O₃particles were hvdrophobized via surface functionalization (>1 hr in 10μl/ml trimethylcholorosilane in CH₂Cl₂). The “PRS” cell utilized ana-SiN_(x):H passivation layer to minimize surface recombination and toserve as an anti-reflection coating, a Ag back reflector to prevent theloss of incident illumination into the growth substrate, and Al₂O₃particles to scatter light incident between the Si microwires. Followingthe inclusion of the selected light-trapping elements, each wire arraywas filled to the tips of the wires with mounting wax (a transparent,non-conducting, thermoplastic polymer). Indium tin oxide (ITO) (120-150nm thick) was then sputtered through a shadow mask to form a top-contactpad and to define individual cells. For both the Scatterer and PRS solarcells, the 80 nm Al₂O₃ particles were observed to form micron-sizedagglomerates that were located near the base of the wires, permittingthe infill region to be conceptually divided into an upper and lowerregion, for which the density of Al₂O₃ particles was significantly lowerwithin the lower infill region. In the PRS solar cells, the 1000 nmthick Ag back reflector covered the growth substrate and the taperedbase of the wires.

Under simulated AM 1.5G illumination, the champion PRS solar cellexhibited markedly higher photovoltaic performance than the championScatterer and As-Grown solar cells, as a result of a significantincrease in short-circuit current density (J_(SC)) brought about by thecombination of light-trapping elements exemplifying embodiments of thepresent disclosure. The champion PRS solar cell produced an open-circuitvoltage (V_(OC)) of 498 mV, J_(SC) of 24.3 mA/cm², and a fill factor(FF) of 65.4%, for an efficiency of 7.92%. The champion Scatterer andAs-Grown solar cells exhibited efficiencies of 5.64% and 3.81%,respectively, with similar V_(OC) and FF but lower J_(SC): 16.6 mA/cm²and 11.8 mA/cm², respectively. The improved efficiencies of the Scatterand PRS cells over the As-Grown cells exemplify the benefits tophotovoltaic performance afforded by the light-trapping structures ofthe present disclosure.

A device, such as the one of FIG. 13 or FIG. 14, includes one or moreconductive layers, wherein at least some portion of each conductivelayer is in contact with one or more of the semiconductor structures orwires (650), whose conductivity provides a path for current collectionfrom the photovoltaic device. The device further includes one or more ofthe following features: a reflective structure, such as the backreflector (115, 620), an antireflective structure, such as the coating(118, 660), a light-scattering structure, such as the light scatteringparticles (123, 640), or a light-concentrating structure, such material(116). Additional reflective, antireflective, light-scattering, orlight-concentrating structures are applied above or below the conductivelayers.

Moreover, according to further embodiments, in such device (600) atleast 80% of the carriers excited by the absorption of light within thesemiconductor material are collected as current from the photovoltaicdevice. Additionally, conductive layers can be located above and belowthe array of elongated semiconductor structures. Moreover, thesemiconductor material occupies less than 10% of the volume of thestructure enclosed between the two conductive layers.

According to further embodiments, the conductive layer comprises one ormore of Ag, In, Al. Each conductive layer can be either at least 90%transparent or at least 90% reflective at a visible wavelength of light.According to further embodiments, some or all of the infill materialcomprises a conductive layer for the photovoltaic device. According tofurther embodiments, the antireflective coating or infill materialserves to passivate some or all of the surfaces of the semiconductorstructures. Some or all of the surfaces of the semiconductor structuresare coated by one or more additional layers to provide passivation ofthe semiconductor surfaces or to increase the conductivity of thephotovoltaic device. Moreover, the reflective material may also serve asa conductive layer.

According to a further embodiment, the present disclosure includes aphotoelectrochemical device comprising, for example, any one of thestructures (101), (102), (103), (104), (105), (106), (107), (108),(109), (1001), (1002), (1003), (1004), (1005), (1006), (1007), (1008),(1009) and (1010) or any combination of such structures. In particular,as mentioned above, FIG. 15 shows a device (700) including an array ofwires (110) and a liquid electrolyte solution (1114). The liquidelectrolyte solution (114) can include any one of the liquid substancesused as infill material, for example, in the structures (101), (102),(103), (104), (105), (106), (107), (108), (109), (1001), (1002), (1003),(1004), (1005), (1006), (1007), (1008), (1009) and (1010) disclosed inthe previous paragraphs.

The examples set forth above are provided to give those of ordinaryskill in the art a complete disclosure and description of how to makeand use the embodiments of the disclosure, and are not intended to limitthe scope of what the inventors regard as their disclosure.Modifications of the above-described modes for carrying out thedisclosure may be used by persons of skill in the art, and are intendedto be within the scope of the following claims. All patents andpublications mentioned in the specification may be indicative of thelevels of skill of those skilled in the art to which the disclosurepertains. All references cited in this disclosure are incorporated byreference to the same extent as if each reference had been incorporatedby reference in its entirety individually.

It is to be understood that the disclosure is not limited to particularmethods or systems, which can, of course, vary. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting. As used in this specification and the appended claims, thesingular forms “a,” “an,” and “the” include plural referents unless thecontent clearly dictates otherwise. The term “plurality” includes two ormore referents unless the content clearly dictates otherwise. Unlessdefined otherwise, all technical and scientific terms used herein havethe same meaning as commonly understood by one of ordinary skill in theart to which the disclosure pertains.

A number of embodiments of the disclosure have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the presentdisclosure. Accordingly, other embodiments are within the scope of thefollowing claims.

LIST OF CITED REFERENCES

-   [1] Kayes, B. M., Atwater, H. A. & Lewis, N. S. Comparison of the    device physics principles of planar and radial p-n junction nanorod    solar cells. J. Appl. Phys. 97, 114302-114311 (2005)-   [2] Kelzenberg, M. D. et al. Predicted efficiency of Si wire array    solar cells. 34th IEEE Photovoltaic Specialists Conference 1-6    (2009).-   [3] Kayes, B. M. et al. Growth of vertically aligned Si wire arrays    over large areas (>1 cm2) with Au and Cu catalysts. Appl. Phys.    Lett. 91, 103110-103113 (2007).-   [4] Marion, B. et al. Validation of a photovoltaic module energy    ratings procedure at NREL. Report No. NREL/TP-520-26909, (1999).-   [5] Aspnes, D. E. in Properties of crystalline silicon (ed Robert    Hull) 677 (INSPEC, IEE, 1999).-   [6] Tiedje, T., Yablonovitch, E., Cody, G. D. & Brooks, B. G.    Limiting efficiency of silicon solar-cells. IEEE Trans. Electron    Devices 31, 711-716 (1984).-   [7] Yablonovitch, E. Statistical ray optics. J. Opt. Soc. Am. 72,    899-907 (1982).-   [8] Kelzenberg, M. D., Putnam, M. C., Turner-Evans, D. B.,    Lewis, N. S. & Atwater, H. A. Predicted efficiency of Si wire array    solar cells. 34th IEEE Photovoltaic Specialists Conference 1-6    (2009).-   [9] Altermatt, P. P., Yang, Y., Langer, T., Schenk, A. & Brendel, R.    Simulation of Optical Properties of Si Wire Cells. Photovoltaic    Specialists Conference, 2009. PVSC '09. 34th IEEE 1-6.

The invention claimed is:
 1. A substrate; A tiled array of micron sizedelongated semiconductor elements extending from the substrate and havinga space between each elongated semiconductor element, wherein thesemiconductor elements have diameters of at least 1 micron and an aspectratio greater than 1; An infill material comprising polydimethylsiloxanelocated in the space between the elongated semiconductor elements; and Aplurality of light scattering elements comprising voids, bubbles,dielectric particles, or metal particles included in the infill materialand surrounding the elongated semiconductor elements, wherein theconcentration of light scattering elements is higher in proximity to theelongated semiconductor elements, increasing the probability ofabsorption into the elongated semiconductor elements or wherein theinfill material includes an infill bottom zone closed to the bottom ofthe structure than to an infill top zone, and the concentration of lightscattering elements is higher in the infill bottom zone than in theinfill top zone, the bottom of the structure being opposite to a zoneexposed to incident light, such that light that is reflected orscattered upwards has more distance with which to interact with thewires; Wherein the array is tiled according to an ordered latticepattern.
 2. The structure of claim 1, wherein the light scatteringmaterial uniformly surrounds all sides of the elongated semiconductorelements.
 3. The structure of claim 2, wherein the infill materialincludes an infill bottom zone closer to a bottom of the structure thanto a infill top zone, and wherein the concentration of the lightscattering material is higher in the infill bottom zone than in theinfill top zone, the bottom of the structure being opposite to a zoneexposed to incident light.
 4. The structure of claim 2, wherein thedielectric composition comprises one element selected from the groupincluding Al₂O₃, BaSO₄, TiO₂, SiO₂.
 5. The structure of claim 2, whereinthe particle of metal comprises one element selected from the groupincluding Ag, Au, Ni, Al and Cu.
 6. The structure of claim 3, whereinthe elongated semiconductor elements are at least 20 microns in lengthand have an aspect ratio of at least 5:1.
 7. The structure of claim 3,wherein the array of elongated semiconductor elements has a packingfraction less than 5%.
 8. A solar cell comprising the structure of claim2.
 9. A photoconverter device, comprising: a top transparent contact, abottom metal contact, and the structure of claim 2 interposed betweenthe top transparent contact and the bottom metal contact.